The operation of sewage treatment plants results in the generation of sizable quantities of sewage sludge. Sludge management constitutes a major portion of waste treatment plant costs. Landfilling, incineration and land application are the major routes for sludge disposal. Hence, there is a clear need for the development of innovative technologies or approaches for maintaining cost-effective disposal and/or recycling options.
Land application of "acceptable" sludge or biosolids is expanding and gaining wider acceptability because of its fertilizer value, and the economics involved. As such, the nutrient content of sludge can be exploited through its adoption as a low-cost fertilizer, and more globally through the conservation of energy and mineral resources (Hall J. E. 1995. "Sewage Sludge Production, Treatment and Disposal in the European Union", J. CIWEM 9:335-343).
However, the heavy metal content of sewage sludges is considered to be a major threat to human health and the environment. The accumulation of these metals in soils can cause plant toxicity, ground water and surface water contamination, or the transfer of increased amounts of metals from plants to humans and animals via the food chain (Torrey S. 1979. "Sludge Disposal By Landspreading Techniques", Noyes Data Corp., N.J., U.S.A.; Jacobs L. W. 1981. "Agricultural Applications of Sewage Sludge", in Sludge And its Ultimate Disposal, J. A. Borchardt et al. (eds.), Ann Arbor Science Publishers Inc., Anne Arbor, Mich., U.S.A.). In the 1980s, more than 50% of the sludges produced in Ontario and in the United States failed to meet the regulatory criteria for acceptable land application because of high metal concentrations (Lue-Hing C, Zeng D. R., Sawer B., Guth E. and Whitebloom S. 1980. "Industrial Waste Pretreatment and EPA Cadmium Limitations", J. Water Pollut. Control Fed. 52:2538-2551).
As a result of source control, heavy metal concentrations in sludges have decreased markedly (Lue-Hing C., Matthews P., Namer J., Okuno N and Spiizosa L. 1996. "Sludge Management In Highly Urbanized Areas", Wat. Sci. Tech. 34:517-524). However, the rate of reduction has decreased as contributions from diffuse sources have become increasingly important. With such sources difficult to control, land application of sludges is still limited (Isaac R. A. and Boothroyd Y. 1996. "Beneficial Use of Biosolids: Progress In Controlling Metals", Wat. Sci. Tech 34: 493-497; MAFF. 1993. "Review Of The Rules For Sewage Sludge Application To Agricultural Land: Soil Fertility Aspects Of Potentially Toxic Elements", Report of the Independemt Scientific Committee, Ministry of Agriculture, Fisheries and Food, London, U.K.).
Most of the metal contamination in sewage sludge is associated with the solids fraction thereof. Thus, decontamination and removal require that the metals be first solubilized. Various chemical methods for the solubilization of metals from sludge have been investigated. These include: acidification (Hayes T. D., Jewell W. J. and Kabrick R. M. 1979. "Heavy Metal Removal From Sludges Using Combined Biological/Chemical Treatment", in Proceedings 34th Industrial Waste Conference, Purdue University, Ann Arbor, Mich., U.S.A.); chemical chelation (Bloomfield C. and Pruden G. 1975. "The Effects of Aerobic and Anaerobic Incubation On the Extractabilities of Heavy Metals in Digested Sewage Sludges", Environ. Pollut. 8:217); liquid ion exchange (Cornwell D. A. and Westerhoff G. P. 1980. "Extract Heavy Metals Via Liquid-Ion Exchange", Water and Wastes Eng. 17:36); chlorination (Olver J. W., Kreye W. C. and King P. H. 1975. "Heavy Metals Release By Chlorine Oxidation of Sludges", J. Water Pollut. Control Fed. 47:2490); and oxidative acid hydrolysis (Kliff. R.J. and Brown S. 1977. "The Development Of An Oxidative Acid Hydrolysis Process For Sewage Sludge Detoxification", in Proc Int. Conf. Heavy Metals in the Environment, Amsterdam, The Netherlands). Wong and Henry (Wong L. and Henry J. G. 1988. "Bacterial Leaching of Heavy Metals From Anaerobically Digested Sludge", in Biotreatment Systems Vol. II, D. L. Wise (ed.), CRC Press, Florida, U.S.A.) compared these five methods and concluded that none was suitable for full-scale application because of cost and operational constraints, and in some cases, unacceptable removal efficiencies.
A biological process for metal solubilization exploiting the presence of indigenous sulphur oxidizing bacteria that use elemental sulphur as their energy source has been developed (Blais J. F., Tyagi R. D. and Auclair J. C. 1992. "Bioleaching of Metals From Sewage Sludge by Sulfur-Oxidizing Bacteria", J. Environ Eng. 118: 690-707; Rick K. K. 1993. "Biological Solubilization of Metals From Anaerobically Digested Sludge in A Semicontinuous System", M.A.Sc. Thesis, Dept. of Civil Engineering, Univ. of Toronto, Toronto, Canada), and has been the subject matter of several patents (U.S. Pat. Nos. 5,217,615; 5,454,948). Moreover, the process seems to be cost-effective with no loss of fertilizer value in the decontaminated biosolids product (Wong L. and Henry J. G. 1988. "Bacterial Leaching of Heavy Metals From Anaerobically Digested Sludge", Biotreatment Systems (D. L. Wise, Ed.), CRC Press Inc., Boca Raton, U.S.A., p. 125; Tyagi R. D., Couillard D. and Tran F. T. 1990. "Studies On Microbial Leaching of Heavy Metals From Municipal Sludge", Water Sci. Tech. 22:229)
However, studies have shown that about 40-60% of the sulphur added is not oxidized during the process (Tyagi R. D., Blais J. F., Meunier N. and Kluepfel D. 1993. "Biolixiviation Des Metaux Lourds Et Stabilisation Des Boues D'epuration; Essai En Biorecteur Opere En Mode Cuvee", Can. J. Civ. Engrg. 20: 57-64). Environmental problems such as soil acidification could occur on land application of the decontaminated biosolids due to acid generation from this residual sulphur (Janzen H. H. and Bettany J. R. 1987. "Oxidation of Elemental Sulfiur Under Field Conditions in Central Saskatchewan", Can. J. Soil Sci. 67: 609-618; Motowicka--Terelak T. and Gador J. 1986. "Effect of Contamination with Sulphur on Soil Properties and Crop Yields in A Lysimetric Experiment. I. Effect of Elemental Sulphur on the Chemical Properties of Soils", Pamietnik pulawski: 7-23).
With respect to the commercial application of such processes, there exist a number of other deficiencies. These include: (a) the shortcomings of the batch process; (b) the difficulty in separating the leached biosolids from the metal contaminated liquid; (c) the detrimental effects of excess sulphur addition; and (d) the large volume that must be leached when all the sludge from a wastewater treatment plant is included.
These problems are further described in the following sections.
(a) Batch Process
Prolonged aeration of sludge in a batch or sequencing batch bacterial leaching process may result in the conversion of ammonia nitrogen into nitrite nitrogen and not nitrate nitrogen as might be expected. Since nitrite is toxic to sulphur oxidizing bacteria, leaching efficiency decreases dramatically, making the process ineffective.
(b) Liquid/Solids Separation
After metal solubilization, the contaminated liquid and the leached biosolids must be separated, and the biosolids thickened and dewatered. This is a weak link in the leaching process. Wong and Henry (Wong L. and Henry J. G. 1984. "Decontaminating Biological Sludges For Agricultural Use", Water Sci. Technol. 17:575) have demonstrated that simple gravity settling is inadequate and that centrifagation is the only practical dewatering method capable of achieving the necessary 20% solids concentration needed for maximizing fertilizer value. Unfortunately high centrifugation costs make this process economically unattractive.
(c) Sulphur Addition
Elemental sulphur is the essential energy source for the sulphur oxidizing bacteria As such, the amount provided in any process is critical. Too little and leaching proceeds slowly resulting in nitrite formation and inhibition of the leaching bacteria By contrast, if excess sulphur is added to hasten acidification, and thus avoid nitrite formation in the batch or sequencing batch process, the residual sulphur in the decontaminated sludge can create acid conditions in the agricultural land on which the sludge is applied.
This problem has been identified in U.S. Pat. Nos. 5,454,948. 5,454,948 relates to a semi-continuous (i.e. sequencing) batch process. In the example presented in that patent, sulphur oxidation efficiencies varied between 6.2% and 56.2%, leaving excessive amounts of residual sulphur in the metal decontaminated sludge. U.S. Pat. No. 5,454,948 suggests the use of sulphur pellets or "immobilized sulphur" in order to facilitate recovery and recycling of excess residual sulphur. However, the process of preparing "immobilized sulphur" is complicated and the economics involved were not addressed.
Residual sulphur in decontaminated sludge is not totally undesirable. After nitrogen and phosphorus, sulphur is the third most important nutrient for plants and crops. However, the sulphur content in decontaminated sludge needs to be controlled so that it is in balance with the other nutrients.
(d) Sludge Type and Quantity
Most research on heavy metal decontamination of sewage sludge has been based on the total sludge volume, both primary and secondary, removed from wastewater treatment processes. Traditionally, this ensured that all metal contaminated sludges were included in the leaching process. Different sludge types, namely, primary, secondary, digested, etc., obtained from various treatment plants have been evaluated for metal solubilization (Blais J. F., Tyagi R. D. and Auclair J. C. 1992. "Bioleaching of Metals From Sewage Sludge By Sulfur-Oxidizing Bacteria", J. Environ. Eng. 118:690-707). However, no comparison has been made between the different sludge types obtained from the same treatment plant for their relative suitability for metal removal by the biological solubilization process.
Therefore, there is a clear need for an improved process for the biological decontamination of metal-laden sewage sludge which addresses these deficiencies in the prior art.